PROMOTER SWITCH IN THE ESCHERICHIA-COLI PTS OPERON

The ptsH operon of Escherichia coil is controlled by two promoters P0 and P1, each of which is regulated by cyclic AMP receptor protein (CRP ) complexed with cAMP (CRP.cAMP). We have studied the in vitro as well as in vivo transcriptional regulation of these two promoters. Each pr omoter exhibits a switching mechanism in vitro, where, depending upon the presence or absence of CRP.cAMP, transcription is initiated from d ifferent start sites termed a and b.P0 (P0a) is affected by supercoili ng: when the template is linear, transcription initiation is switched to a site 3 base pairs upstream (P0b) and becomes more CRP.cAMP depend ent. Transcription from the P1 promoter (P1a) switches initiation site s to 7 base pairs downstream (P1b) in the presence of CRP.cAMP. Most t ranscription in vivo was from P1a, and P0b could not be detected in vi vo. Glucose has independent positive effects on pts expression in vivo . The results indicate that the two different regulatory mechanisms (o ne through CRP.cAMP, the other through glucose) are working together f or fine control of pts expression.